[ntp:questions] Re: Clock accuracy & auto setting : digital television does a crap job of providing time services...
John Ackermann N8UR
jra at febo.com
Mon May 1 13:32:14 UTC 2006
Max Power wrote:
>>>LF time services are OK, and are necessary over large transnational
>>>regions -- like Sub Saharan Africa, Australasia and South America ... but
>>>any new LF service needs to be more technologically advanced than WWVB,
>>>or DCF77 and its Swiss twin. In these regions 10 LF frequencies need to
>>>allocated, but the signal to be transmitted needs to be more modern than
>>>WWVB or DCF77 -- maybe using some form of low complexity PSK or low
>>>complexity QAM and 240 hz to 480 hz of bandwidth. The signal must be
>>>futureproofed -- as above.
One thing to remember is that LF stations were originally designed, and
still serve, another important purpose: to provide precise *frequency*
measurement capability. The original intention of WWVB was to provide a
stable carrier frequency in a spectrum that has minimal ionospheric and
propagation disturbances. The original WWVB receivers were
phase-tracking units that allowed a local frequency standard to be
directly compared to the USFS.
As in timekeeping, GPS has become the method of choice for frequency
calibration, but there are still people using WWVB (and presumably other
standard frequency stations) for their original purpose. If nothing
else, it provides direct traceability to NBS without needing to adjust
for NIST<-->USNO offsets.
One of the advantages of the current modulation scheme is that it
doesn't mess with the carrier phase. If more complex modulation schemes
reduce or destroy the ability to phase track the carrier, one of the
primary purposes of the LF stations will be lost.
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